This invention relates to an improved printed circuit board for mounting electrical components. More particularly, this invention relates to a printed circuit board having bond pads for soldering a component to the board, which bond pads are formed in an opening of a polymeric coating overlying the board and are elevated above the coating to facilitate soldering operations.
It is known to manufacture a microelectronic package that comprises a microelectronic component, such as an integrated circuit die, mounted onto a printed circuit board by plurality of solder bump interconnections. For this purpose, a plurality of solder bumps are affixed to a face of the component. The board comprises a copper circuit trace that includes bond pads. For assembly, the component is arranged with the printed circuit board such that each solder bump rests on a corresponding bond pad on the board. The assembly is heated to a temperature sufficient to melt the solder bump, whereupon the molten solder is reflowed and wets the bond pad. Upon cooling, the solder resolidifies and bonds to the bond pad to complete the solder bump interconnection.
During reflow, there is a tendency for the molten solder to spread along the circuit trace, which, if uncontrolled, depletes solder from the desired interconnection and leads to catastrophic collapse of the component against the board. Therefore, a solder mask is applied to the board to confine the molten solder to the bond pad. A suitable solder mask comprises a coating formed of a polymeric material that is not wettable by molten solder. Openings are formed in the coating to expose the bond pads. As a result, the bond pads are recessed beneath the surface of the coating. It is then necessary to accurately position the component so that each bump is inserted within the corresponding microscopic opening and rests against the bond pad, which is essential for forming a strong solder bond. Moreover, it is found that stresses within interconnections formed in this manner are poorly distributed, thereby leading failure to of the solder bond and shortening the useful life of the package. Thus, there is a need for a printed circuit board that comprises bond pads having bonding surfaces that lie above the polymeric coating to facilitate the positioning of the component and reduce stress within the solder connections, thereby improving the reliability of the solder bonds and extending the useful life of the package.